The field of the invention is nuclear magnetic resonance (NMR) methods and systems. More particularly, the invention relates to the compensation of NMR systems for variations in the polarizing magnetic field.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. An NMR signal is emitted by the excited spins after the excitation signal B1 is terminated, and this signal may be received and processed.
When utilizing these NMR signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is required that the polarizing field B0 be stable during the series of imaging sequences, which may have a duration of a few seconds to fifteen minutes. The required stability is quite high, and typically changes of from 0.1 to a few parts per million (ppm) can degrade the spectra or image. The stability requirement is thus quite severe, and is especially difficult to achieve in resistive and permanent magnets, as opposed to superconducting magnets.
The instabilities may be caused by external disturbances, (e.g. moving ferrous masses such as elevators), and imperfections in the magnet system. In resistive magnets such imperfections include instabilities in the magnet current and thermal contraction of the coils. In permanent magnets the ambient temperature affects both the dimensions of the magnet and the flux produced by its material and thus this type of magnet is quite sensitive to thermal fluctuations.
The polarizing field stability requirement has been addressed in U.S. Pat. No. 4,623,843 of Macovski, and U.S. Pat. No. 4,417,209 of Hounsfield. They teach how to measure the presence of unwanted fluctuations in the polarizing field xcex94B0 and how to use the measured signal to compensate for xcex94B0. This xcex94B0 measurement is done by NMR, using a separate reference sample located outside the imaged object but inside the B0 field region. The reference sample can be excited separately or by the same RF excitation field as that of the object. The reference signal from the sample produces an NMR signal having a frequency f0+xcex94f0, which is proportional to B0+xcex94B0. The measured frequency changes xcex94f0 are used for demodulating the acquired NMR signals received from the object, thus compensating for the xcex94B0 instabilities during reception. Methods for compensating the magnitude of the polarizing field B0 during excitation of the object are also described. In addition to the cost of added B0 sensors and associated circuitry, the signal-to-noise (S/N) ratio of the NMR sensor is at best only a little better than needed and at worst it can degrade the final image quality.
As disclosed in U.S. Pat. No. 5,488,950, one solution to these problems is to employ electron-spin resonance (ESR) as a means for measuring changes in the polarizing magnetic field. As with NMR, ESR employs a strong polarizing magnetic field to measure resonant signals, but in ESR the resonant signals are produced by electrons rather than nuclei. ESR typically operates at microwave frequencies and employs electronics and coil structures that are substantially different than those employed in NMR. One difficulty with this solution is the substantial cost of the additional ESR circuitry.
Another approach is disclosed in U.S. Pat. No. 4,970,457 which interleaves the imaging pulse sequences performed during the scan with pulse sequences that do not employ phase encoding. The echo signals produced by these interleaved pulse sequences are Fourier transformed and the peak is detected and used as a measure of the polarizing field B0. While this method detects changes in the magnitude of the polarizing field in the region of the imaged slice, it does not provide any information regarding B0 gradients produced by the polarizing field across the imaging volume.
When performing functional magnetic resonance imaging (fMRI), pulse sequences having long transmit repeat (TR) times are employed and multiple NMR echo signals are acquired. Such pulse sequences (e.g. echo planar imaging (EPI) sequences) are characterized by low SNR signals acquired at a very low band width per pixel in the phase encoding direction of the image. These are particularly sensitive to phase errors caused by non-uniform fields and the like. They are repeated a number of times during a study to acquire a series of images depicting the brain over a time course. It has been discovered that even small movements of a subject""s head during an fMRI study can disturb the B0 polarizing field sufficiently to produce warping and artifacts in acquired fMRI images. These perturbations of the polarizing field are localized within the subject and include not only changes in the polarizing field magnitude throughout the imaging volume, but also changes in that magnitude as a function of position within the imaging volume. In other words, B0 gradients are produced across the imaging volume.
The present invention is a method for performing an MRI scan in which shim navigator NMR signals are acquired during the scan that enable changes in the polarizing magnetic field to be measured throughout the duration of the scan. The measured polarizing field changes may be employed to prospectively correct acquired NMR data by changing one or more scan parameters such as the imaging pulse sequence gradient waveforms.
An object of the invention is to provide a shim navigator pulse sequence which acquires a navigator NMR signal that enables the changes in the B0 field along any arbitrary axis within the imaging volume to be measured. Each shim navigator pulse sequence acquires two NMR echo signals, and after each is fast-Fourier transformed, the phase difference between the two is calculated to provide an indication of the spatially dependent polarizing field (B0). The measured B0 field is compared with a reference, or baseline, value obtained at the beginning of the scan to determine the change in polarizing magnetic field.
Another object of the invention is to correct for changes in the polarizing magnetic field that occur in the subject being imaged during a scan. By acquiring shim navigator NMR signals read out along three orthogonal axes, motion-induced changes in the polarizing field B0 in any arbitrary direction(s) within the subject can be measured across the entire imaging volume.